Technetium 99m is a short-lived, radioisotope which has been found useful in radioscanning various organs of the body. Heretofore, the radioactive material has been available in colloidal form associated with sulfur. A mode of preparation is disclosed in "Preparation, Distribution and Utilization of Technetium 99m Sulfur Colloid" by Stern, H. S., McAffee, J. G. and Subramanian, G., Journal of Nuclear Medicine, 7, 665 to 675, (1966). An improvement in this basic technique is disclosed in U.S. Pat. No. 3,683,066 to Ascanio, et al. In order for the technetium 99m to be absorbed for the brief time necessary for radioscan measurements to be made it has been found necessary to administer the radio isotope in the form of a colloid as set forth in the aforementioned references. It has been found, however, that while the heretofore known colloidal compositions are useful in radioscanning certain organs, such as the liver, the colloids are not noted in other organs which should, theoretically, be examinable by this technique such as, for example, the lymph glands. There appears to be a general consensus of medical opinion that the reason for the non-appearance of the known colloids in these organs is due to a size distribution of the major component of the colloids lying above 0.1 microns in diameter.
It would therefore be desirable to provide Technetium 99m colloids whose radioactivity is substantially equal to that of the heretofore available colloids while having a size distribution wherein the major portion of the particles is 0.1 microns or less.
It should be noted that accurate measurement techniques in this particle size area which involves light scattering measurements are expensive and not readily available. The smallest practical readily available ultrafilter will block the passage of particles greater than 0.1 micron in diameter. It is known, however, that a Sephadex G-25-80 gel column will only permit the passage of colloidal material substantially smaller than 0.1 microns in diameter. The exact size range of these particles has not been determined, however, all have the ability to pass through a 0.1 micron ultrafilter. This practical though by no means arbitrary method is utilized herein as the criterion of size measurement.
All of the work done heretofore on technetium 99m colloids has assumed that it was necessary for the sulfur to be associated with the technetium 99m. It was further generally assumed, though never clearly proved, that the technetium was in some way deposited upon the surface of the colloidal sulfur and hence clearly the sulfur was an essential and irremovable part of the colloidal system.
It has also been known (see for example U.S. Pat. No. 3,683,066, column 2, lines 23 through 25) that the presence of a gelling agent such as gelatin is important as a growth inhibiting and colloid stabilizing reagent. In addition to the named gelatin, other agents, including bovine serum albumin, are known as suitable agents however heretofore there has been no reason to consider one of these agents superior in action to any other.